EP0240190A2

EP0240190A2 - Process for manufacturing ceramic sintered bodies and mold to be used therefor

Info

Publication number: EP0240190A2
Application number: EP87302072A
Authority: EP
Inventors: Isao Oda
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 1986-03-31
Filing date: 1987-03-11
Publication date: 1987-10-07
Also published as: EP0240190A3; DE3768300D1; EP0240190B1; JPS62227603A

Abstract

The process of the invention comprises pressure molding a ceramic material (B) with a mold (1, 2, 3, 4). at least a part (1) of which consists of a low melting alloy having a melting point temperature between 40°C and 250°C, preferably between 46°C and 180°C, said molding being carried out at a temperature lower than the melting point of the low melting alloy, thereafter heating the mold to melt and remove the low melting alloy, and subsequently firing the thus obtained molded body into a sintered body. The invention is advantageously applied to the manufacture of sintered bodies having a complicated shape, e.g. having a hollow of which the inside dimension is larger than its entrance. The low melting alloy part is especially a centre core part. The use of a low melting alloy mold part achieves high dimensional accuracy in the ceramic body, allows high molding pressure and provides a simple process.

Description

The present invention relates to a process for manufacturing ceramic sintered bodies and a mold to be used in the manufacturing process, particularly, a process for manufacturing ceramic sintered bodies wherein ceramic sintered bodies with a complicated configuration can be obtained by making use of a mold comprising a low melting alloy, and the mold to be used for the manufacturing process.
Owing to superiorities of ceramic sintered bodies over metals, such as heat resistance, heat insulating properties, low thermal expansibility, hardwearing properties, corrosion resistance, etc., extensive trials have been made recently in applications of the sintered bodies, particularly, to automobile engine parts, etc.
In such cases, engine parts, etc. often have a complicated shape and, particularly in the case where, for example, a ceramic body designed to have a hollow portion wider than its entrance is manufactured, generally the following processes and the like have so far been adopted:

a process which comprises molding a hollow body by using a straight (cylindrical) center core, followed by scraping the inside wall of the molded hollow body before firing, to widen the inside hollow space, and
② a process which comprises molding by using a split center core, followed by releasing the split core.

Further, a process for manufacturing a mold itself by using low melting alloys, as shown in the gazette of Japanese Patent Application Laid-open No. 59-82,142, and a process for manufacturing a plaster mold by using low melting alloys, as described in the gazette of Japanese Patent Application Laid-open No. 60-259,407, have recently been proposed.
However, the above-mentioned conventional processes CD and ② have disadvantages, such as a complicated process, difficulties in precision molding, etc.
Further, in either process described in the gazette of Japanese Patent Application Laid-open No. 59-82,142 or 60-259,407, the low melting alloys are used as a manufacturing means of the mold and not as a component of the mold.
, As a result of assiduous researches, in view of the difficulties as mentioned above in the conventional processes, the inventor has paid attention to low melting alloys which have not hitherto been used as a component of a mold, and has succeeded to obtain ceramic sintered bodies with a more complicated shape, by fabricating and utilizing a mold comprising a low melting alloy component.
Namely, the present invention provides a process for manufacturing ceramic sintered body, which comprises pressure molding a ceramic material with a mold at least a part of which consists of a low melting alloy having a melting point temperature between 40°C and 250°C, thereafter removing the low melting alloy by heating to obtain a molded body, and subsequently firing the molded body into a sintered body, and also provides a mold to be used in said manufacturing process of ceramic sintered bodies, which comprises, in at least a part thereof, a low melting alloy having a melting point temperature between 40°C and 250°C.
Embodiments of the invention will now be described with reference to the accompanying drawings, wherein:

Fig. 1 illustrates an example of a piston cavity, Fig. la being its plan view and Fig. lb being a vertical sectional view taken along the plane X-X of Fig. la;
. Fig. 2 is a vertical sectional view showing an example of a center core made of a low melting alloy, according to the invention;
Fig. 3 is a vertical sectional view illustrating an example such that a low melting alloy center core and ceramic powder are covered by a mold;
_Fig. 4 is a schematic vertical sectional view illustrating an example of a molded body carrying a low melting alloy center core;
_Fig. 5 is a vertical sectional view illustrating an example of a mold for a combustion chamber;
Fig. 6 is a slant view showing another example of the low melting alloy center core;
_Fig. 7 is a schematic vertical sectional view illustrating that a center core and other mold components are combined and ceramic powder is filled in the formed mold;
Fig. 8 is a schematic vertical sectional view showing a further example of a molded body carrying a center core;
Fig. 9 is a vertical sectional view showing further different example of a mold;
Fig. 10 is a vertical sectional view illustrating a mold for injection molding incorporated, as its part, with a center core; and
Fig. 11 is a sectional view showing a molded body of an exhaust port liner.

The low melting alloy to be employed in the mold according to the invention preferably comprises, as its main ingredient(s), at least one metal selected from the group consisting of lead, tin, bismuth, indium and cadmium, and particularly preferred are those comprising lead, tin or bismuth as the main ingredients.
The low melting alloys are those having a melting point temperature between 40°C and 250°_C, preferably between 46°C and 180°C. Because, if a low melting alloy having a melting point lower than 40°C is used, there will be a fear of its melting out during the molding step, according to the ambient temperature in the molding, and on the other hand, if a low melting alloy of a melting point exceeding 250°C is used, there will be a fear of collapsing of the ceramic molded body during the melting step of the low melting alloy, due to the difference in thermal expansion between the low melting alloy and the ceramic molded body. Further, the reason why the temperature range between 46°C and 180°C is preferred is that melting operations of the low melting alloy in the above range are readily feasible.
Ceramic materials to be employed in the present invention are not particularly limited and various ceramic materials are employable among which SiC, Si₃N₄, Zr0₂, cordierite, magnesium-alminium-silicate, etc. are generally used.
Further, when pressure molding the ceramic materials by using a mold, it is preferred to be carried out by means of any one of press molding, hydrostatic pressure molding, press-cast molding and injection molding, with a molding pressure of generally 10 kg/cm2, or more and preferably 100 kg/cm² or more. The reason why the molding pressure be 10 kg/cm² or more is that, if the molding pressure is lower than 10 kg/cm2, the molded body will have such a low compressed density that it is prone to disintegrate or break off,_'and so a satisfactory molded body will not be able to be obtained. Further, the reason why 100 kg/cm2 or more pressure is preferred is that particularly when, as a molding process, the press molding, hydrostatic pressure molding or injection molding is selected, a dense molded body can be obtained.
According to the process of the invention, at the outset, a precision casting or precision finish is applied to a mold of which at least a part is made of a low melting alloy. Then, using this mold as a master form, a ceramic material is pressure molded whereby an objective shape is replicated to the ceramic material.
The above pressure molding is performed at a mold temperature lower than the melting point temperature of the low melting alloy employed, i.e., in most cases a room temperature is applied. During the molding, therefore, since the low melting alloy used in the mold is kept solid and hard, as the mold temperature is lower than the melting point of the low melting alloy, even a pressure molding with a considerably high molding pressure results in no shape deformation of the mold caused by pressure transformation.
After that, removing the low melting alloy by heat-melting, a ceramic molded body with a complicated shape is obtained, which is then fired to produce the objective ceramic sintered body.
The present invention will be explained in more detail hereinafter by way of examples which are not intended to restrict the invention. In the examples, "percent" and "part" mean by weight unless otherwise designated.

Example 1

A low melting alloy having a melting point temperature of 46.7°C and a composition of 44.7_% Bi, 22.6_% Pb, 8.3% Sn, 5.3% Cd and 19.1% In, was melted in a dryer at 60°C, poured into a mold of 100 mm dia. x 30 mm length and solidified by cooling down, to produce a lump of the low melting alloy. As a part corresponding to piston cavity A as shown in Fig. 1, a low melting alloy center core 1 as shown in Fig. 2 was formed by precision machinework from the lump.
On the other hand, as a ceramic material, 100 parts of Si₃N₄ powder having an average particle dia. of 1 pm were incorporated with, as sintering aids, 3 parts of MgO, 2 parts of SrO and 4 parts of Ce0₂, and mixed homogeneously. To 100 parts of the thus prepared ceramic powder, 60 parts of water were added to form a slip which was then dried by a spray dryer, yielding molding powder of 60 µm average particle dia. which was used as molding material B.
Then, in order to mold a piston cavity, as shown in Fig. 3, low melting alloy center core 1 was fixed by means of rubber stopper 3 in cylindrical rubber mold 2 having a predetermined form for molding a piston head, the space between rubber mold 2 and center core 1 was filled with the above-prepared ceramic material B which was introduced from an opening of rubber mold 2, and the opening was plugged up with rubber stopper 4. Then, a pressure of 5 tons/cm² was applied by means of hydrostatic pressing, and molded body 5 carrying low melting center core 1 as shown in Fig. 4 was obtained.
Further, ceramic molded body 5 carrying low melting alloy center core 1 was finished by scraping the periphery of the molded body into a predetermined shape (such as shown by the broken line in Fig. 4), and then introduced into a dryer having its temperature maintained at 60°C, to melt away low melting alloy center core 1. Through subsequent firing in N₂ atmosphere at 1,700°C for 30 minutes, the objective piston cavity sintered body was obtained. No defects, such as stains, cracks, etc., of the low melting alloy were observed in anywhere of this piston cavity. Besides, a desired shape was able to be formed without any deformation.
From two portions 6 and 7 of the resulted sintered body, as shown in Fig. lb, respective testpieces of 1.5 mm x 2 mm x 20 mm size were cut out and tested for flexural strength and density. As a result, it was confirmed that an article of high strength and uniform quality with a density not lower than 95% of the theoretical density was able to be obtained, as shown in Table 1 below.
Further, the melted out low melting alloy was reused for the same molding process as described above and the objective article was able to be obtained with no difficulties.

Example 2

A low melting alloy having a melting point temperature of 150°C (a melting range of 176-145°C) and a composition of 12.6% Bi, 47.5% Pb and 39.9% Sn, was melted in a dryer at 180°C and the molten low melting alloy was casted, according to a process used in a precision casting, into casting mold 8 of a combustion chamber shown in Fig. 5, and after cooling, casting mold 8 was splitted along opening surface 17, to release center core 9 as shown in Fig. 6.
Besides, an epoxy resin was casted in the above- described casting mold 8, and after curing, the casting mold was released to obtain center core 10. From a result of comparison of the outside diameters of the obtained center cores 9 and 10 with that of the prototype mold for the casting mold, it was found that the low melting alloy center core was superior in dimensional accuracy to the epoxy center core.
On the other hand, 100 parts of SiC powder having an average diameter of 0.5 pm were incorporated with 3 parts of B₄C and 3 parts of carbon, as sintering aids, and mixed homogeneously.
Then, in order to mold a combustion chamber, using low melting alloy center core 9 as a part of molds 11, 12 and 13, as shown in Fig. 7, these molds were combined together, filled with the above-prepared ceramic material C, and closed by setting top mold 11 which was then pressed downwards with 100 kg/cm2 pressure. Likewise, using epoxy resin center core 10, a molding according to a process similar to the above was carried out and ceramic molded body 14 carrying center core 9 or 10 as shown in Fig. 8 was obtained.
Further, to remove the center core, ceramic molded body 14 carrying the low melting alloy was introduced into a dryer at 180°C, to melt out the low melting alloy. Additionally, ceramic molded body 14 carrying the epoxy resin was heated at the temperature elevation rate of 10°C/hr. under an inert atmosphere and maintained at 500°C for 3 hours to remove the resin.
As a result, the molded body produced by making use of the low melting alloy had a high dimensional accuracy and was free from cracks. In contrast, one produced by making use of the epoxy resin had cracks and deformation as well as a low dimensional accuracy, so that a good product could not be obtained.
Further, the molded body obtained by making use of the low melting alloy was covered with latex rubber and then pressurized, by means of hydrostatic pressure molding, with 2.5 tons/cm2 pressure, followed by maintaining under Ar gas atmosphere at 2,100°C for 30 minutes, and the objective combustion chamber sintered body was obtained.

Example 3

A low melting alloy having a melting point temperature of 95°C and a composition of 52.0% Bi, 32.0_% Pb and 16_% Sn was melted in a dryer at 120°C and casted into a matrix as shown in Fig. 9, to produce center core 15. The center core 15 was incorporated in mold 16 for injection molding and set as a part of the mold as shown in Fig. 10.
On the other hand, as an injection molding material, 4.3% MgO, 37.8% A1₂0₃, 50.4% Ti0₂, 5.5% Fe₂0₃ and 2.0% Si0₂ were mixed together to provide ceramic powder. Then, to 100 parts of the above-prepared powder, were added 3 parts of methylcellulose, 1 part of stearic acid and 3 parts of water, as injection molding aids, and from the resulted mixture, as a material for an exhaust port liner, an extrusion of 60 mm dia. and 200 mm length was obtained by means of a deairing pug mill. This extrusion was then put into a cylinder of an injection molding machine and injection molded in the aforementioned, previously prepared mold for injection molding shown in Fig. 10, with an injecting pressure of 1.5 tons/cm² and a mold temperature of 30°C.
Then, as conditioning the moisture by means of a humidity controlling dryer maintained at 65°C, the molded body was kept at 65°C for 24 hours to be cured and dried, subsequently the temperature was raised, by means of an electric oven with internal air circulation, at the rate of 10°C/hr. from room temperature up to 120°C. Maintaining it at 120°C, the low melting alloy was melted out, the temperature was raised again at the rate of 10°C/hr., then it was kept at 500°C for 5 hours to remove binders, and a molded body of exhaust port liner as shown in Fig. 11 was obtained.
Subsequently, through firing at 1,500°C for 5 hours, the objective exhaust port liner sintered body was obtained. No defects were found in any portion of this exhaust port liner.
As was explained above, since a low melting alloy is used in a mold according to the process of the invention, no mold deformation will occur during pressure molding and, moreover, the low melting alloy can be readily melt-removed by heating, so that ceramic sintered bodies with a complicated shape can be obtained.
Further, a mold comprising a low melting alloy is easily recoverable and reproducible and, therefore, very economically advantageous.
While there have been shown and described the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various alterations and modifications thereof can be made without departing from the scope of the invention as defined by the following claims.

Claims

1. A process for manufacturing a ceramic sintered body, which comprises pressure molding a ceramic material with a mold at least a part of which consists of a low melting alloy having a melting point temperature between 40°C and 250°C, thereafter removing the low melting alloy by heating to obtain a molded body, and subsequently firing the molded body into a sintered body.

2. A process as claimed in claim 1, wherein said melting point temperature is between 46°C and 180°C.

3. A process as claimed in claim 1 or 2, wherein said pressure molding is performed at a mold temperature lower than the melting point temperature of the low melting alloy.

4. A process as claimed in claim 1,2 or 3, wherein the pressure molding is carried out with a molding pressure of at least 10 kg/cm2.

5. A process as claimed in any one of claims 1 to 4, wherein the pressure molding is carried out by means of any one of press molding, hydrostatic pressure molding and injection molding.

₆. A process as claimed in any one of claims 1 to 5, wherein the low melting alloy comprises, as its main ingredient(s), at least one metal selected from the group consisting of lead, tin, bismuth, indium and cadmium.

7. A mold to be used in the process claimed in claim 1, which comprises, in at least a part thereof, a low melting alloy having a melting point temperature between 40°C and 250°C.

8. A mold as claimed in claim 7, wherein the low melting alloy comprises, as its main ingredient(s), at least one metal selected from the group consisting of lead, tin, bismuth, indium and cadmium.